In a wireless communication system having a wireless base station (BTS: Base Transceiver Station) that provides a wireless communication area (cell), a wireless terminal (MS: Mobile Station), and a radio network controller (RNC) that controls the BTS, the MS can communicate with other MSs via the BTS if the MS is within a range in which the MS can exchange radio waves with that BTS.
In such a wireless communication system, the MS generally measures the communication qualities of the communications with the BTS with which the MS is communicating and with other BTSs located close to the MS, and periodically or occasionally reports information on the communication quality to the RNC via the BTS. The RNS includes a function that controls cell (path) switching for switching from the BTS with which the MS is communicating to another BTS when the RNC detects that the communication quality with the BTS with which the MS is communicating drops to equal to or smaller than a predetermined threshold due to migration of the MS or the like, based on the report (communication quality information) from the MS.
In the meantime, in the High Speed Downlink Packet Access (HSDPA) that is one of wireless communication techniques, a single shared resource is dynamically time-division multiplexed and code multiplexed, and allocated to a plurality of MSs for the purpose of increasing the peak speed and reducing transmission delay in the downlink (here, the communication direction from a BTS to an MS). In addition, in order to improve the transmission efficiency, techniques, such as the adaptive modulation and coding (AMC) or the hybrid automatic repeat request (HARQ), are applied.
In addition, in the HSDPA, a High Speed Downlink Shared Channel (HS-DSCH) is used as a transport channel that terminates at a BTS. It is noted that this channel is applied in a Packet Switch (PS) domain.
During an HSDPA communication, in order to share one channel among multiple users, and to avoid congestion and buffer overflow on an MS side, a flow control is performed between Iur and Iub. Here, “Iur” is an interface used between RNCs, and “Iub” is an interface used between an RNC and a BTS. It is noted that such a flow control is performed using the High Speed Downlink Shared Channel Frame Protocol (HS-DSCH FP) which is a control frame on the user plane (U-plane).
One example of the entire operations in a wireless communication system to which the above-described HSDPA is applied will be described with reference to FIG. 6 to FIG. 10. FIG. 6 is a sequence diagram illustrating the communication operation between an RNC and a BTS in a wireless communication system to which the above HSDPA is applied. In addition, FIG. 7 is a diagram illustrating a frame format of a capacity request signal, and FIG. 8 is a diagram illustrating setting values of a capacity request signal. Furthermore, FIG. 9 is a diagram illustrating a frame format of a capacity allocation signal, and FIG. 10 is a diagram illustrating setting values of a capacity allocation signal. It is noted that a capacity request signal is a control signal for requesting the transmission rate of data sent from an RNC to a BTS. In addition, a capacity allocation signal is a response signal for the capacity request signal, and a control signal used by the BTS to inform the RNC of the transmission rate.
As depicted in FIG. 7 and FIG. 8, the frame format of a capacity request signal is configured to include a Frame CRC field that indicates a CRC of the frame, an FT field that indicates whether the frame is a data frame or a control frame, a Frame Type field that indicates the frame type of the frame (for example, “0x0A”), a Spare bit that is used as a spare, a CmCH-PI field that indicates a priority of the user data, a User Buffer Size field that indicates the amount of data buffer waiting for transmission of user data, and a Spare Extension field that is a spare.
In addition, as depicted in FIG. 9 and FIG. 10, the frame format of a capacity allocation signal is configured to include a Frame CRC field that indicates a CRC of the frame, an FT field that indicates whether the frame is a data frame or a control frame, a Frame Type field that indicates the frame type of the frame (for example, “0x0B”), a Spare bit that is used as a spare, a CmCH-PI field that indicates a priority of the user data, a User Buffer Size field that indicates the amount of data buffer waiting for transmission of user data, a Max MAC-d PDU Length field that indicates the maximum MAC-d PDU length that can be sent, an HS-DSCH Credits field that indicates the allowable MAC-d PDU number sent by the RNC, an HS-DSCH Interval field that indicates a valid interval (period) of the above HS-DSCH credit, an HS-DSCH Repetition Period field that indicates continuous valid interval of the above HS-DSCH credit, and a Spare Extension field that is a spare.
In a wireless communication system to which the HSDPA is applied, the flow control as depicted in FIG. 6 is performed using a capacity request signal and capacity allocation signal described above. FIG. 6 is a sequence diagram illustrating the communication operation between an RNC 100 and a BTS 200 in a wireless communication system to which the above HSDPA is applied. It is noted that a definition of an HS-DSCH FP is standardized in order to archive a flow control between the RNC100 and the BTS 200.
In this flow control, firstly, the RNC 100 as the transmission side determines that user data transmission rate (hereinafter, sometimes simply referred to as “transmission rate”) to the BTS 200 is 0 kbps, and that there is user data to be transmitted for a predetermined time period, the RNC100 sends a capacity request signal (FT=1, CmCH-PI=N (the range of N is between 0 and 15), and the user buffer size=U>0) to the BTS 200 as the reception side (see (1) in FIG. 6).
In response to receiving the above capacity request signal from the RNC100, the BTS 200 sends a capacity allocation signal (FT=1, CmCH-PI=N, the PDU length (>0), credit (>0), and the interval (>0)) for specifying the transmission rate at the RNC100 to the RNC100(see (2) in FIG. 6). Here, the transmission rate is set to “transmission rate: high,” for example.
Next, in response to receiving the above capacity allocation signal, the RNC100 sends user data (HS-DSCH data frame) at the transmission rate specified by the BTS 200 (“transmission rate: high”) to the BTS 200 (see (3) in FIG. 6). It is noted that the particular queue to send the capacity request signal and the capacity allocation signal described above is not stipulated, and is set by the user to any appropriate timing. For example, when the transmission rate is needed to be changed due to some reason, such as an increase or decrease the number of users communicating on the same channel, for example, the BTS 200 may autonomously specify the transmission rate based on capacity allocation signal to change the transmission rate without receiving a queue from the transmission side (capacity request signal) (see (4) and (5) in FIG. 6, for example). Similar to this, the RNC100 may also autonomously determine the transmission rate without complying with the transmission rate specified by the BTS 200 side (capacity allocation signal).
Thus, the flow control as described above is adapted to control the transmission rate efficiently where a bottle neck is located on a transmission path between the RNC 100 and the BTS 200, for example.
Next, cell switching control in the above-described wireless communication system will be explained with reference to FIG. 11 and FIG. 12. FIG. 11 is a schematic diagram illustrating cell switching control in the wireless communication system, and FIG. 12 is a sequence diagram pertaining to the cell switching control.
As depicted in (1) in FIG. 11, for example, it is assumed that an MS 300 is carrying out wireless communication with a BTS 200-1 in the cell of the BTS 200-1 (the solid black area in (1) in FIG. 11). At this time, the MS 300 periodically or occasionally measures the communication quality with the BTS 200-1 and the communication quality with another BTS 200-2 that is present in the proximity to the MS 300, and reports measurement results of those communication qualities to the RNC100 via the BTS 200-1 (200-2). It is noted that the measurement processing is performed using a control plane (C-plane) depicted with a dotted line allow in FIG. 11, and communication of user data is performed using the user plane (U-plane) depicted with a solid line allow in FIG. 11.
The RNC 100 determines whether or not there is any other cell that has a better communication quality than the current cell based on the above report (communication quality measurement result) from the MS 300.
At this time, when the RNC 100 determines that a better communication quality is obtained if the cell of the BTS 200-2 is used than when using the cell of the BTS 200-1 as for data communication with the MS 300, for example, switch cell (path) change (cell switching control) to change the cell with which the MS 300 communicates to the cell of the BTS 200-2 is performed to change from the communication state depicted in (1) in FIG. 11 to the communication state depicted in (2) in FIG. 11 (the black solid area in (2) in FIG. 11).
Here, the cell switching control as described above will be further explained with reference to the sequence diagram in FIG. 12. Similar to the example depicted in FIG. 11, the example depicted in FIG. 12 also illustrates the case in which the RNC 100 performs cell switching control from the cell of the BTS 200-1 to the cell of another BTS 200-2.
Firstly, the MS 300 measures the communication quality at the cell of the BTS 200-1 with which the MS 300 is communicating and communication at the cell of another BTS 200-2, and sends the measurement results to the RNC100 as wireless communication qualities (event ID: 1D).
The RNC 100 determines whether or not to perform cell switching control based on the wireless communication qualities received from the MS 300. In the example depicted in FIG. 12, for the data communication with the MS 300, it is determined that a better communication quality is obtained if the cell of the BTS 200-2 is used than when the cell of the BTS 200-1 is used, the RNC 100 determines whether or not cell switching control is performed to switch the cell to communicate with the MS 300 from the cell of the BTS 200-1 (cell change origin) to the cell of the BTS 200-2 (cell change destination) (see “commence cell change”).
Next, when determining to perform cell switching control, the RNC 100 sends an RL reconfiguration preparation message (RL RECONF. PREPARE) to the BTS 200-2 for preparing a Radio Link (RL) configuration of the cell change destination BTS 200-2. In response to receiving this, the BTS 200-2 replies to the RNC 100 with an RL reconfiguration ready complete message (RL RECONF. READY) that is a reply therefor. In addition, the RNC 100 sends an establishment request message (ESTABLISH REQUEST) to the BTS 200-2. In response to receiving this, the BTS 200-2 replies to the RNC 100 with an establish confirm message (ESTABLISH CONFIRM) that is a reply therefor (see the processing depicted with the reference symbol (a)).
Furthermore, the RNC 100 sends an RL reconfiguration preparation message (RL RECONF. PREPARE) to the BTS 200-1 for preparing an RL configuration of the BTS 200-1. In response to receiving this, the BTS 200-1 replies to the RNC 100 with an RL reconfiguration ready complete message (RL RECONF. READY) that is a reply therefor (see the processing depicted with the reference symbol (b)).
At this time, the RNC 100 starts the timer (see “setting timer”), and sends an RL reconfiguration commit message (RL RECONF. COMMIT) and a physical channel reconfiguration message (PHY. CH. RECONF) to which the activation time is embedded to the BTS 200-1, the BTS 200-2, and the MS 300, respectively (see the processing depicted with the reference symbol (c)). At this time, the activation time is used as a timer value until carrying out an actual cell switching as described in Non-Patent Reference 1 that will be mentioned later, and when the above-described timer expires a timer value that is set to the activation time (hereinafter, may be simply referred to as “timer value), cell switching control is performed by the RNC 100.
On the other hand, the BTS 200-1 sends a capacity allocation signal (transmission rate: 0 (credit=0 and interval=0)) to the RNC 100 which is a flow control signal for suspending transmission of downlink data (user data) from the RNC 100 (see the processing depicted with the reference symbol (d)).
In response to receiving the above capacity allocation signal from the BTS 200-1, the RNC 100 suspends transmission of user data to the BTS 200-1, and waits for carrying out of cell switching control until the above activation time comes. On the other hand, the BTS 200-1 continues to send user data remaining in the local station 200-1 to the MS 300 until the above timer value expires.
After the above timer value expires, the RNC 100 performed configuration of the intra-apparatus path (within the RNC 100, from the terminating apparatus to the BTS 200-2 to the terminating apparatus relating to the HSDPA communication) (see the processing depicted with the reference symbol (e)).
On the other hand, after the above timer value expires, the BTS 200-2 sends a capacity allocation signal (transmission rate: X (MAC-d, the PDU length, credit, and interval>0)) to the RNC 100 which is a flow control signal for resuming communication of downlink data (user data) from the RNC 100 (see the processing depicted with the reference symbol (f)).
In response, the RNC 100 detects completion of the above processing (see the processing depicted with the reference symbols (e) and (f)), receives an RL physical channel reconfiguration complete message (PHY. CH. RECONF. COMPLETE) indicating reconfiguration complete report from the MS 300 to resume downlink data transmission to the BTS 200-2, while exchanging a release request message (RELEASE REQUEST) and a release confirm message (RELEASE CONFIRM) with the BTS 200-1, and disconnecting the channel with the BTS 200-1 (see the processing depicted with the reference symbol (g)).
In other words, when determining to perform cell switching control based on the report on the wireless communication quality (1D) between the MS 300 and the BTS 200, the RNC 100 sets the activation time (timer value), and performs cell switching control from the BTS 200-1 (cell change origin) to the BTS 200-2 (cell change destination) after waiting for expiration of the timer value.
It is noted that the above-described cell switching control is disclosed in Non-Patent References 2 and 3 that will be listed below, and Patent References 1-4 listed below describe technique related to cell switching control methods in wireless communication systems containing an HSDPA.
The following Patent Reference 1 discloses determining whether or not any data is remained in an old node, and suspending data transmission from an RNC to old node based on the determination result. In addition, the following Patent Reference 2 discloses execution of a negotiation of the activation time. Furthermore, the following Patent References 3 and 4 disclose flow control signal processing between a serving RNC (S-RNC), a drift RNC (D-RNC), a node B, and a user equipment (UE) upon handover.    Patent Reference 1: Japanese Translation of PCT International Application No. 2005-510950    Patent Reference 2: International Patent Publication No. WO2004/057887    Patent Reference 3: Japanese Translation of PCT International Application No. 2005-525057    Patent Reference 4: Japanese Translation of PCT International Application No. 2005-521360    Non-Patent Reference 1: 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; Mobile radio interface layer 3 specification; Radio Resource Control (RRC) protocol; lu Mode (Release 7) (3GPP TS44.118 V7.1.0 (2005-07)    Non-Patent Reference 2: 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; High Speed Downlink Packet Access; Iub/Iur protocol aspects (Release 5) (3GPP TR25.877 V5.1.0 (2002-06))    Non-Patent Reference 3: 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; High Speed Downlink Packet Access (HSDPA); Overall description; Stage 2 (Release 7) (3GPP TS25.308 V7.1.0 (2006-12))
In the meantime, cell switching control is not performed until the timer value expires in the above-described cell switching control method.
However, by the time that the timer value expires, the BTS 200-1 that is the cell change origin may complete transmission of user data to the MS 300. Such a case may pose a problem in that, although there is not user data to be transmitted to the MS 300 left within the BTS 200-1, no cell switching control is performed until the timer value expires. As a result, the communication efficiency of the wireless communication system may be deteriorated since there is time during which there is no user data to be transmitted from the BTS 200-1 to the MS 300 (no communication time).
In addition, due to change (deterioration) of the communication environment, the BTS 200-1 that is the cell change origin may not successfully complete transmission of user data to the MS 300 by the time that the timer value expires. Such a case may also pose a problem in the cell switching control is not performed until the timer value expires although the BTS 200-1 cannot successfully send user data to the MS 300. As a result, transmission delay of the user data may occur.